Project Summary Lyme disease has become a worldwide public health problem and is now the most common tick-borne infection in the United States and Europe. To better understand how Borrelia burgdorferi, the causative agent of Lyme disease, persists in the infected tick and mammal, many laboratories have focused on identifying and characterizing outer surface proteins from this spirochete. The interface between B. burgdorferi and its tick or mammalian host is its outer surface; therefore, outer membrane proteins (OMPs) localized to the surface of this organism play an important role in virulence and disease. Along these lines, in the last funding period we focused on characterizing the outer membrane protein system that exports OMPs to the surface of B. burgdorferi. This OMP export complex, termed the ?-barrel assembly machine (BAM), is comprised of three major outer membrane proteins in B. burgdorferi, the OMP BamA along with two accessory lipoproteins, BamB and BamD. A second OMP transport system has recently been identified in some dual-membraned bacteria (i.e., bacteria with both an inner membrane and an outer membrane), which has been named the translocation and assembly module (TAM) system. The TAM system consists of the OMP designated TamA and a single inner membrane (IM) protein designated TamB. While the BAM system exports the majority of OMPs, the TAM system is now known to export various autotransporters and other virulence-associated OMPs. During our studies characterizing the borrelial BAM export complex, we also identified a hypothetical borrelial protein, BB0794, which we determined is a borrelial TamB ortholog. Interestingly, almost all dual-membraned organisms characterized to date have been found to encode a TamB ortholog, but only the Proteobacteria contain the complete TAM system comprised of both TamA and TamB. The functional role of TamB in all dual-membraned organisms lacking TamA, including B. burgdorferi, has therefore remained an unanswered question. Our recent studies, however, may have finally answered this question as we discovered that the borrelial TamB protein specifically interacts with BamA of the BAM complex. Consistent with TamB being involved in OMP export and outer membrane biogenesis, we observed that a TamB mutant has altered cellular morphology and increased sensitivity to antibiotics. The combined findings lead us to propose a new functional role for TamB orthologs in dual-membraned organisms, such as spirochetes. The underlying hypothesis of our proposed studies is that a better understanding of the OMP export systems in B. burgdorferi will lead to the identification of novel virulence factors and vaccine candidates for Lyme disease. Importantly, the studies we propose in this application could also change the current paradigm of OMP export and outer membrane biogenesis in many bacterial pathogens.